1. Field of the Invention
The present invention relates to a film formation method of a thin film. Further, the present invention relates to a manufacturing method of a semiconductor device using the film formation method.
2. Description of the Related Art
As for existing film formation methods, PVD (physical vapor deposition) using physical means such as vacuum deposition, sputtering, or ion plating; CVD (chemical vapor deposition) using chemical reaction of energy such as heat, plasma, or light; and formation by spraying or coating such as surface polymerization, a sol-gel process, or electroplating are given as typical examples.
The film thickness of a film composing a semiconductor device is from several nm to approximately 1μm. The chemical vapor deposition or sputtering is frequently used for the formation methods of the thin film.
As a typical example of a thin film used for a semiconductor device, a conductive film or various kinds of insulating films for insulating each wiring are used. A silicon nitride film is further given as a typical example of such an insulating film. A silicon nitride film is effective in preventing penetration of moisture or alkali ions. Therefore, a silicon nitride film is employed for a coating film of a substrate, or a protective film of a semiconductor element.
As a film formation method of an insulating film, plasma CVD is employed. The plasma CVD is generally used because of the advantages of high step coverage, high moisture resistance, and ability to form a film at a low temperature (Reference 1: Japanese Patent Laid-Open No. 8-274089).
A film formed by plasma CVD, typified by a silicon nitride film has relatively high hydrogen content. This is because ammonia other than silane and nitrogen is used for a reactive gas.
Thus, in the film having high hydrogen content, hydrogen is desorbed from the film by heat or application of voltages. As a result, troubles of change in characteristics such as a change in the threshold or acceleration of deterioration is caused in a semiconductor element or a semiconductor device using the semiconductor element.
Therefore, a process of “dehydrogenation” in which the film formed by plasma CVD is heated at a high temperature is required. As shown in Reference 1, a method for forming a film in the state where the decomposition of a source gas is controlled is proposed to form a film with low hydrogen content even without the dehydrogenation.
Further, a technique that makes it possible to control the hydrogen content of a silicon nitride film by controlling the decomposition of ammonia is described in Reference 1.
When a film is formed by normal CVD, heating at a high temperature is required. On the other hand, plasma CVD can form a film at a relatively low temperature (around 400° C.); however, heating at a temperature higher than 480° C. is required to reduce the hydrogen content in the film. In this case, when a film formed from a low heat-resistant material, typified by an organic resin such as acrylic, or aluminum wirings are applied to a base film, another film cannot be formed thereover by plasma CVD.
When a film is formed by sputtering, a source gas and a target can be selected depending on the characteristics of the film. For example, silicon or silicon nitride should be employed as a target, and nitrogen, argon, or nitrogen and argon should be employed for a sputtering gas thereby forming the silicon nitride film with a low content of hydrogen. A contaminant (generally, hydrogen of ammonia (NH3) used in plasma CVD) from a source gas can hereby be reduced.
Further, a film can be formed at a low temperature by sputtering, so that it can be formed directly on an organic resin substrate or an organic resin member each of which has low heat resistance.
However, sputtering has the following problems: (1) difficulty and high cost of forming a target for a large substrate, (2) decline in productivity which is caused by the maintenance carried out in a chamber open due to impracticability of self-cleaning in the chamber, (3) impurities in an atmosphere that can easily contaminate a film when a film is formed, (4) impurities in a target that contaminates a product.